Automatic power transmission mechanisms for automative vehicles presently in use usually comprise a single pressure source in a form of an engine driven pump. The pump develops a circuit pressure that is used by the control valve sytem for the automatic transmission to supply the control valve circuit pressure. A typical automatic control valve system using an engine driven pump in this manner may be seen by referring to U.S. Pat. Nos. 3,309,939 and 3,446,098, which are assigned to the assignee of this invention.
Portions of the control valve system of conventional automatic transmissions require high pressure and other portions require lower pressures. For example, the circuit pressure required to energize clutch and brake servos in an automatic transmission mechanism require high pressure, especially when the torque delivered by the transmission is at an elevated level. On the other hand, the lubrication points in the transmission, as well as the hydrokinetic torque converter, require lower pressures. The use of a single pump makes it necessary for the pump to operate at the highest pressure that is required to maintain the clutches and brakes at the necessary pressure level. Thus the pump operates with a maximum pressure differential across the pump at all engine speeds. That is, it is operated at a pressure differential across the pump that is sufficiently high to satisfy the requirements of the portion of the circuit having the greatest pressure requirement. This reduces the transmission efficiency since the maximum pressure must be maintained even though the flow needed at each particular pressure may be a small portion of the total flow.
In the case of an infinitely variable transmission employing adjustable sheaves and drive chains or belt, a high pressure is required to maintain sufficient clamping pressure on the sheaves while the other components of the circuit require lower pressures. The pump power required in such infinitely variable transmission systems, as in all transmission, equals the pressure times the volume flow rate. Thus a single fixed displacement pump is required to absorb more power from the engine than otherwise would be necessary. The same would be true with a variable displacement pump in such an environment because all of the flow from the pump is at maximum pressure.
An example of an infinitely variable transmission having a pump that distributes pressure to the clamping servos, to the ratio control servo and to the various elements of the control valve system may be seen by referring to U.S. Pat. No. 3,115,049. That patent also is assigned to the assignee of this invention.